1. Field of the Invention
The invention relates generally to the use of measuring instruments for chemical analysis and more specifically to the use of an instrument which uses the voltage and current characteristics of a sampling of a solution to determine the concentrations of various materials in the solution, i.e., a voltammetric instrument.
2. Description of the Related Art
A wide variety of techniques and devices are currently used to analyze the concentrations of various analytes in a solution. One such technique is voltammetry. Voltammetry is an electrochemical method for quantitative and qualitative analysis based on the relationship between a changing applied voltage and the resulting current through the solution being analyzed.
Like other chemical analysis techniques, voltammetry of solutions causes a background value (signal) upon which the response of the analyte of interest is superimposed. If the technique employed entails a systematic change in a controlled parameter such as wavelength or electrode potential, generally there is a different background value at each value of the controlled parameter, i.e., a background curve.
U.S. Pat. No. 4,628,463 to Starrock et al., for RAPID-SWEEP ELECTROCHEMICAL DETECTOR FOR CHEMICAL ANALYSIS OF FLOW STREAMS, discloses a data acquisition system with some ability to obtain accurate response peaks. However, the system is limited at low analyte concentrations by the background signal.
For many years, a technique known as background subtraction has been used to correct the measured values after data acquisition. In a typical background subtraction technique, raw data is collected with a measuring instrument, converted from analog-to-digital form, and passed to a digital computer, wherein the computer can subtract a previously or subsequently measured background value from the present measurement. While background subtraction is a useful approach, it is not adequate when the background curve changes over a wide range and the analyte response, i.e., the response of interest, is small in comparison to the change in the background. In such a situation, it has been necessary to operate the instrument or measuring device at low sensitivity, and the analyte response is then so small that it cannot be measured with adequate precision, if it can be detected at all.
This problem is especially troublesome in digital instruments in which an analog-to-digital converter (ADC) is used to convert the signal to a digital value to make it compatible with a computer. In such cases, the dynamic range of the instrument is limited by the number of bits of the ADC and possibly by the number of bits of the computer bus and processor. For example, a 12-bit ADC has only 4096 response levels (i.e., levels of precision). Almost all of these levels might be used by the changing background curve (the background value), and only a very few bits (levels) of precision might be used for the small analyte response. Thus, the measured analyte response is poorly defined and imprecise.
One way to address this situation is to use an ADC with more bits. However, this solution can be very costly and can inconveniently require a longer computer word (data) format, resulting in slower processing times and the requirement of more computer memory for storage capacity.
Thus, it can be seen that a need yet exists for a voltammetric instrument with a large dynamic range, without requiting the use of a larger data format. It is to the provision of such that the present invention is primarily directed.